In my current Romani campaign, I am fighting a lot of barbarians, and I have discovered a good way to deal with unarmoured troops. I use a historical army makeup of 1 general,1 equites exraordinarii, 1 campanian cav, 2 triarii, 4 principes, 4 hastati, 4 velites, and several mercenary units. These would probably be longswordsman, as you are based in Gaul and near Gaul. As my strategy depends in a large part on javelins, it is imperative that you claim the high ground. Put your units in the checkerboard formation, with the velites filling in the gaps between the hastati. Put your mercenary longswordsmen in a good flanking position. Have all units except for them have fire at will on. Put your cavalry in good flanking positions. If your general in very experienced, consider using him to flank also. The enemy will most likely charge your center. At this point, make certain that the velites have skirmish mode off and defence mode on. As the enemy draws near, your velites will start throwing javelins. The hastati will join in, and then the principes. At this point the enemy has probably already engaged your velites. Don't have them retreat. Have them keep throwing javelins into the fray. After all of the hastati and principes have thrown their pila, tell them to charge. You can commit the triarii at this point also. The combination of javelin hail+massive charge should have inflicted massive morale penalties. In weaker armies, a rout may have started. If not, flank the enemy with your mercenaries and cavalry, and charge. Victory is yours.

As this tactic relies heavily on javelins, I'd is only suitable for battlesagainst lightly armored foes.